Filler

From EirtakuWiki

Filler is material in a show that exists for the sole purpose of delaying a pre-existing, prevalent plot. Typical filler consists of short throw-away stories or arcs that have no effect on the ongoing continuity.

Filler is often created when the writers of a show run out of source material, and are forced to start creating plots on their own. This often happens when manga-based anime reach the end of an existing and ongoing manga, and the creators do not wish to diverge from the manga continuity. An example of this is Naruto, which has entire series of filler, and a counter-example Fullmetal Alchemist where on reaching the final extant manga volume, the anime directors decided to diverge completely from the manga continuity. A major disadvantage to the creators of this sort of filler is that the writers do not know what is going to happen in the future of the continuity, and so cannot plan around it. This adds to the throw-away feel to many filler episodes.

Another form of filler is the use of recap episodes, which are commonly used to allow animators to rest or catch up with demanding TV-show schedules.